feat: added get_seconds_since_epoch()

Fixes #1.

README.md

@@ -110,7 +110,9 @@ Method scripts are a little more complicated than adapter scripts, as they need
 3. A function `adjust_card(response, data, difficult) -> [..., bool]`, which takes in the user's response to a card (guaranteed to be one of the ones you defined in `const RESPONSES`), the card's data, and whether or not it is marked as difficult. It should return the new data (this is where you update the properties that you use to determine a card's weight) and whether or not the card should now be marked as difficult. Note that the meaning of 'difficult' is entirely method-dependent, and it is simply one of the ways Forne lets users see how they're doing with their sets.
 4. A function `get_default_metadata() -> ...`, which should return the default values you want to use for a card's `data`.
 
-As an example to help you understand all this a bit better, here's a very naive learning method with heavy commenting:
+If your method depends on scheduling when a card should next be reviewed, you can get a representation of the time with `get_seconds_since_epoch`, which returns the number of seconds since Unix Epoch (Jan. 1 1970), which will be negative if you've done a bit of time travel.
+
+As an example to help you understand all this a bit better, here's a very naive learning method:
 
 ```rhai
 const RESPONSES = ["y", "n"];

src/lib.rs

@@ -13,6 +13,7 @@ pub use set::*;
 use anyhow::Result;
 use fancy_regex::Regex;
 use rhai::{Dynamic, Engine, EvalAltResult};
+use std::time::SystemTime;
 
 /// A Forne engine, which can act as the backend for learn operations. An instance of this `struct` should be
 /// instantiated with a [`Set`] to operate on and an operation to perform.
@@ -155,6 +156,17 @@ impl Forne {
                 Ok::<_, Box<EvalAltResult>>(Dynamic::from_array(pairs))
             },
         );
+        // Support for working with timestamps
+        engine.register_fn(
+            "get_seconds_since_epoch", // Gets the number of *seconds* since Unix epoch
+            || {
+                match SystemTime::now().duration_since(SystemTime::UNIX_EPOCH) {
+                    Ok(duration) => duration.as_secs() as i64,
+                    // If we're before 01/01/1970...well ok then!
+                    Err(err) => err.duration().as_secs() as i64 * -1,
+                }
+            },
+        );
 
         engine
     }

src/methods/speed-v1.rhai

@@ -1,6 +1,7 @@
 const RESPONSES = ["y", "n"];
 
 fn get_weight(data, difficult) {
+   print(get_seconds_since_epoch());
    return data.weight;
 }
 fn adjust_card(res, data, difficult) {